While most laser sources and more precisely laser diode sources produce an astigmatic beam of light having a substantially non-uniform intensity profile, numerous laser applications require an illumination with a substantially uniform profile. Applications include biomedical applications, such as bio-detection, and precision inspection, such as inspection of microelectronic components, solar cells and others. Some more specific applications require a polychromatic illumination, i.e. with various wavelength components or over range of wavelengths, with a laser line of uniform intensity and width.
U.S. Pat. No. 4,826,299 to Powell, provides a lens for expanding a laser beam along one axis in order to provide a laser line of uniform intensity and width. Such a diverging lens has an acylindrical surface defined by a base curve in the shape of an angle with a rounded apex. The radius of curvature of the acylindrical surface is thus smaller in the center and increases smoothly towards both ends. As described in Powell, the acylindrical surface fits to a base curve defined in a Cartesian coordinate system (x,y,z) by the following equation:
  y  =            cx      2              1      +                        (                      1            -                                          (                                  1                  +                  Q                                )                            ⁢                              c                2                            ⁢                              x                2                                              )                          1          ⁢                      /                    ⁢          2                    wherein c is a curvature constant and Q is a conic constant, and wherein the product Q·c lies between 0.25 and 50 mm−1 and Q is less than −1. The second surface of the acylindrical lens may either be planar or cylindrical.
A Powell lens is an achromatic refractive lens that provides great uniformity along a laser line but it is quite sensitive to the width of the laser beam at its input. For a Powell lens to be used to generate a polychromatic illumination with a uniform profile, the shape and width of the laser beam should be alike for all wavelengths of the laser beam. With refractive beam shaping techniques, a number of laser beams at various wavelengths with alike spatial profiles would need to be combined. This would require a quite complex optical system with many optical elements in precise alignment and using much space.